Why did he say "Hoy Hoy"?
Rashi: It is an expression of announcing to gather. Targum Yonasan of "Yar'em Min Shamayim" (Shmuel II, 22:14) is 'Achlu Min Shemayah.'
Radak: This calls out. It is doubled to strengthen the matter. Also "Hoy Tziyon Himalti" (the coming verse) calls out.
What is the meaning of "v'Nusu me'Eretz Tzafon"?
Rashi: The exiles of My nation should leave from amidst Bavel and Ashur.
Radak: This applies to those who remained in Bavel who did not ascend the first time. They should rush to leave, return to Yerushalayim and build the Bayis, for the time came. The prefix hints to a missing word (Tze'u v'Nusu), like "Nirdam v'Rechev va'Sus" (Tehilim 76:7; it is like Nirdam ha'Melech v'Rechev va'Sus).
Malbim: This tells the exiles in the north to leave from there.
What do we learn from "k'Arba Ruchos ha'Shamayim Perashti Eschem"?
Radak citing Avodah Zarah 10b: Just like the world cannot endure without the winds, it cannot endure without Yisrael 1 .
Radak: I said that they should leave Galus. This is only those who were exiled to Bavel ? "Yosheves Bas Bavel" (11), for the rest are far from them, for I spread them in Galus like four directions. The four corners of the world are far ? east from west, north from south. So Yisrael are far from each other in their Galus, and the time to leave did not yet come.
10b: If it meant that Hashem spread Yisrael to the four corners of the world, it should rather say 'l'Arba Ruchos?'